1. Field of the Invention
The present invention relates to a method for inexpensively manufacturing a sheet having moisture permeability, water resistance and excellent mechanical strength that is used in fields such as sanitary materials, waterproof materials, packaging materials and building wall materials.
2. Description of the Related Art(s)
Porous sheets having moisture permeability and water resistance are used in various fields such as pouches for containing disposable pocket heaters, desiccants, deodorizers and deoxidizers and the like, disposable diapers, clothing, agricultural and horticultural covering materials and architectural materials such as house wraps. The demands being placed on the moisture permeability and water resistance of these sheets for use as house wraps in particular have increased recently.
Although various types of moisture-permeable, water-resistant sheets are used as house wrap materials, since asphalt felt and sheathing boards have insufficient moisture proofing and moisture resistance, they are not satisfactory as house wraps. In addition, flash-spun polyethylene non-woven fabric has excessive gas permeability, which in addition to resulting in inferior heat retention, also results in a complex manufacturing method as well as additional problems including high costs (e.g., Japanese Patent Laid-Open Publication No. 63-286331).
Therefore, although inexpensive porous sheets having moisture permeability and water resistance have been proposed, since many of these porous sheets are added large amounts of organic fillers and drawn in order to form numerous fine holes therewithin, they lack strength and result in significant problems in handling due to inferior tearing strength in particular. Various contrivances have been made to solve this problem, and ways of reinforcing these sheets by laminating coarse mesh fabric or non-woven fabric having superior strength therewith have been examined. In the lamination of the prior art, although extrusion lamination using inexpensive thermoplastic polymer for an adhesive layer is typically used, this method results in a remarkable loss of moisture permeability. As a means of solving this problem, applying an adhesive in the form of dots, applying an adhesive in the form of a matrix, or applying an adhesive in the form of an extremely thin layer (of about 3 .mu.m) has been proposed as in, for example, Japanese Patent Laid-Open Publication No. 63-286331. However, in the case of this method, an expensive adhesive and an adhesive application process are required, which not only invite higher costs, but also make it difficult to always obtain satisfactory results since increasing adhesive strength and retaining moisture permeability are mutually offsetting.
Moreover, when heating is performed to obtain sufficient adhesive strength, problems occur including the occurrence of contraction since the porous sheet is drawn in the manufacturing process, and wrinkles are formed in the product after lamination or the appearance thereof is impaired as a result of remarkable curling. In order to compensate for these shortcomings, the direct pressing of a coarse mesh structure, having a low melting point adhesive layer on its surface, and a gas permeable polyolefin film is proposed in Japanese Patent Laid-Open Publication No. 64-90746. It is described that it is important during pressing that only the mesh structure be heated and be pressed over non-heated gas permeable polyolefin film by a high-temperature roller to prevent loss of gas permeability and moisture permeability by closing the holes of the gas permeable polyolefin film due to heat. In this method, however, adhesive strength is extremely low, and reinforcing effects are not obtained that are sufficient in practical terms.
Thus, in order to solve the above-mentioned problems of the prior art, an object of the present invention is to provide a lamination method of a porous sheet and a coarse mesh reinforcing material that allows a reinforced laminated material that retains moisture permeability and has excellent mechanical strength to be obtained inexpensively without the occurrence of wrinkling or remarkable curling.